1. Field of the Invention
The present invention refers to electrodes for electrolytic capacitors and their production process.
2. Description of the Related Art
It concerns the technical field of materials for electronic devices, especially for electrolytic capacitor applications, which are widely used in electric circuitry applications. Up to the present date, the techniques of manufacturing an electrode for electrolytic capacitors are based on electrochemical processes where the increase of area is obtained by material removal, e.g. by electrochemical etching. However, due to removal of material, the thickness of the substrate in present electrochemical technologies has to be about 100 xcexcm being comparatively thick with respect to miniaturized applications. Further, the production process is expensive since total power consumption for electrochemical processes is high and chemical waste from production creates disposal problems.
Accordingly, it is the object of the invention to provide an improved electrode with high capacitance, which enables the use of thin substrate material, and is cheaper, and avoids chemical waste during production, and a production process thereof.
This object is solved by the present invention providing a process for the production of an electrode for electrolytic capacitors made of a conductive substrate onto which a porous layer of aluminium oxide is deposited on both sides in an at least two-phase process. This process consists of at least a vacuum deposition of unstoichiometric aluminium oxide in a reactive atmosphere and a following surface oxidation process by a treatment under reactive plasma.
With this advanced technology the use of a substrate with thickness between 15 to 30 xcexcm is possible, because the substrate is not weakened by removal of material, but new material is deposited thereon. The thinner substrate provides electrodes with a higher volume capacitance. Further, since the vacuum process does not create any chemical waste there are no disposal problems and this process is environmentally advanced. In addition, the power consumption for the production of the electrode in the vacuum deposition process is less than the total power consumption needed for the electrochemical technology, reaching equal or higher volume capacitance. Thus, the production process according to the present invention is economically convenient. A further advantageous aspect of the invention is the creation of spongy structures showing a high specific area, which are stabilized by a process of plasma oxidation by means of a material addition technique.
In an advantageous development of the present invention, a pretreatment under reactive plasma in an oxidizing atmosphere is applied to the substrate prior to the vacuum deposition of aluminium oxide in order to clean and activate the substrate surface and to enable and enhance reliable attachment of aluminium oxide thereon.
Preferably, the deposition of unstoichiometric aluminium oxide is obtained by a reaction of evaporated aluminium at controlled rate and oxygen at controlled flow, the two sides of the substrate being guided at a predetermined angle with respect to each other and to the cloud of aluminium vapour.
In order to achieve easy handling of the substrate, it is guided in the guide device during the vacuum deposition of aluminium oxide and/or the pretreatment under reactive plasma. The guide device has an unwinding roll from which the substrate is unrolled, a rewinding roll on which the treated substrate is rolled and a plurality of guide rollers defining a mainly angled, spiral or wound spiral guide path for the substrate from the unwinding roll to the rewinding roll and a dome disposed above a source of aluminium vapour, the two inner sides of the dome exposed to the aluminium vapour belonging to the opposed surface sides of the substrate. In this arrangement, both sides of the substrate are automatically exposed to the vapour of aluminium oxide in one process with one aluminium source only.
The following treatment under reactive plasma can be performed according to the present invention in a chamber in an oxygen atmosphere at low pressure, the substrate being guided through said chamber in continuous operation. In this arrangement, only two rolls for unwinding and rewinding the substrate are needed, the substrate being tightened between the two rolls while passing through the chamber.
If the substrate is to be used as anodic electrode the substrate is preferably submitted to the final anodic oxidation process. Such can be performed in a solution of ammonium adipate at a voltage between about 8 and 140 V.
Further, the invention is related to a xe2x80x9cRoll-to-Rollxe2x80x9d-system for the production of an electrode for electrolytic capacitors being disposed in a vacuum deposition chamber having inlet and outlet ports and a guide device with an unwinding roll, a rewinding roll and a plurality of guide rollers defining a mainly angled, spiral or wound guide path for the substrate from the unwinding roll to the rewinding roll and a dome disposed above a source of metallic, e.g. aluminium, vapour, the two inner sides of the dome exposed to the vapour belonging to the opposed surface sides of the substrate.
The present invention is related as well to an electrode for electrolytic capacitors made of a conductive substrate with an unstoichiometric porous layer of aluminium oxide on both sides which is produced in a process described above. The electrode preferably has a layer of aluminium oxide with a dendritic structure.
Other features, advantages and possible uses of the present invention will be discussed in more detail below with reference to the accompanying drawings. It is to be understood that the scope of the present invention is intended to encompass all features described and illustrated independently or in any appropriate combination.